Hydrogel wound dressing for use with suction

ABSTRACT

A wound dressing comprising: an air-impermeable backing sheet having an aperture for attachment of a suction element; an air-permeable screen layer on a wound facing side of the backing sheet; and a substantially air-impermeable hydrogel layer extending across a wound facing side of said screen layer and joined in substantially airtight fashion to a periphery of said backing sheet around said screen layer. Also provided is a wound treatment system comprising a wound dressing according to the invention and a source of suction in fluid communication with said aperture.

This application is a continuation of U.S. patent application Ser. No.14/663,203, filed Mar. 19, 2015, which is a continuation of U.S. patentapplication Ser. No. 13/377,071, filed May 4, 2012, Issued U.S. Pat. No.8,992,509 on Mar. 31, 2015, which claims priority to U.S. National Stageof International Application No. PCT/GB2010/001143, filed Jun. 10, 2010,which claims priority to Great Britain Patent Application No. 0910022.3,filed Jun. 10, 2009, the contents of which are hereby incorporated byreference in their entirety.

The present invention relates to improved wound dressings incorporatinga hydrogel wound contacting layer. The invention also relates to woundtreatment systems incorporating such dressings, and to methods ofmedical treatment with such dressings.

Wound dressings that comprise a continuous, wound-contacting layer of ahydrogel are known. Such dressings exhibit low adhesion to the woundsurface, but the rate at which such dressings can absorb liquid fromexuding wounds is limited. As a result, liquid may pool underneath thesedressings resulting in leakage of liquid from the dressings andmaceration of skin around the wound.

U.S. Pat. No. 5,076,265 and WO-A-03086255 describe methods of makingapertured hydrogel wound contacting layers that are moreliquid-permeable than continuous hydrogel wound contacting layers.However, it will be appreciated that some of the advantages of thecontinuous hydrogel wound contacting layer are impaired by the presenceof apertures.

Vacuum wound dressings, also known as negative pressure wound treatments(NPWT), are known. These dressings apply a reduced pressure continuouslyto the surface of a wound to promote wound healing.

For example, EP-A-0620720 and EP-A-0688189 describe vacuum treatment foraccelerating wound healing. They describe the use of a cover for sealingabout the outer perimeter of the wound, under which a vacuum isestablished to act on the wound surface. This vacuum applied to thewound surface accelerates healing of chronic wounds. A screen ofopen-cell foam material is provided under the cover to provide the spacein which the vacuum is formed and to reduce tissue ingrowth. Sufficientvacuum is applied for a suitable duration to promote tissue migration inorder to facilitate the closure of the wound. Suitable pressures abovethe wound are between about 0.1 and 0.99 atmospheres. The vacuum can besubstantially continuous, wherein the pressure is relieved only tochange the dressing on the wound. Alternatively, the patent teachescyclic application of vacuum in alternating periods of application andnon-application. In a preferred embodiment, vacuum is applied in 5minute periods of application and non-application.

WO-A-0189431 describes vacuum wound dressings wherein the screen furthercomprising a layer of a collagen scaffold material to promote woundhealing. The preferred collagen material is small intestine submucosa(SIS).

WO-A-2005123170 describes vacuum wound dressings comprising activescreen materials that selectively remove undesirable substances fromwound fluid. In certain embodiments, the screen material comprises agel-forming polymer.

WO-A-2009066106 describes vacuum wound dressings that comprise areservoir compartment containing a porous material for absorption ofwound fluid. In some embodiments the screen is separated from theabsorbent layer by a microporous membrane that allows the passage of gasbut not liquids.

US-A-20060189910 describes a vacuum wound treatment device having asponge screen. The wound-facing side of the screen may contain a“growth-enhancing matrix”, which may be “a gel-like growth-enhancingmatrix”. This matrix comprises a cell-growth enhancing substrate that isup to over 90% open space.

US-A-20080215020 describes a vacuum wound dressing having a woundcontacting layer which may be gauze impregnated with hydrophilichydrogel substance. There does not appear to be any disclosure of acontinuous hydrogel layer extending across the dressing in air-tightfashion.

WO-A-2008005532 describes wound contacting sheets for vacuum wounddressings. It appears that all of the wound contacting sheets describedare air-permeable.

WO-A-2007030601 describes vacuum wound dressings having a woundcontacting layer that is said to be typically porous.

In a first aspect, the present invention provides a wound dressingcomprising: an air-impermeable backing sheet having an aperture forattachment to a source of suction; an air-permeable screen layer on awound facing side of the backing sheet; and a substantiallyair-impermeable hydrogel layer extending across a wound facing side ofsaid screen layer and joined in substantially airtight fashion to aperiphery of said backing sheet around said screen layer.

In a second aspect, the present invention provides a wound treatmentsystem comprising a wound dressing according to the invention, and asource of suction in fluid communication with said aperture.

In a further aspect, the present invention provides a method forpromoting wound healing comprising the steps of: applying a wounddressing according to the invention to a wound, connecting the apertureto a source of suction, and creating a reduced (i.e. sub-atmospheric)pressure above said hydrogel layer.

The dressings according to the present invention do not normally applysignificantly reduced pressure directly to the surface of the wound, andtherefore are distinct from the vacuum dressings of the prior art. Thedressings of the present invention apply a reduced pressure to the topsurface of the wound contacting layer of hydrogel, whereby the pressuredifferential between the wound facing surface of the hydrogel and thetop surface thereof tends to draw liquid through the hydrogel layer,resulting in a hydrogel dressing that is able to absorb liquid from anexuding wound faster than previously known dressings with a continuoushydrogel wound contacting layer.

In certain embodiments of the dressing or system according to theinvention, a tube or a coupling is attached to the aperture in thebacking sheet for connection to the source of suction. For example, thedressing aperture may be provided with a push, screw, snap, Luer-lock orbayonet-type fitting for attachment of the suction source.

Suitably, the wound dressing of the invention further comprises asuction manifold in fluid communication with the aperture and locatedinside the screen layer or intermediate the screen layer and the backingsheet. The term “suction manifold” refers to a hollow body having aplurality of apertures for collecting fluid from a plurality oflocations under the backing sheet. The manifold may for example comprisea branched tube which may be perforated, an apertured envelope, or aperforated spiral-wound tube. Other suitable suction manifolds aredescribed in WO2004/037334. The suction manifold may be embedded in thescreen layer, but preferably substantially does not contact the hydrogellayer. The suction manifold delivers suction across the dressing and mayprovide drainage of wound fluid from the screen.

The dressing or system according to the invention may further beprovided with a valve for controlling the application of suction throughthe aperture. In certain embodiments the valve may be closed to maintaina desired atmosphere or pressure in the space above the hydrogel layer,or it may be a one-way or non-return valve to maintain reduced pressureover the hydrogel after removal of the vacuum source.

The backing sheet is formed from a sheet material that is substantiallyor completely impermeable to air. The backing sheet should suitably beformed from substantially gas-impermeable or semipermeable sheetmaterial in order to be able to maintain a reduced pressure in the spacebetween the hydrogel layer and the backing sheet. Thermoplastic sheetmaterials of various types are suitable. The backing sheet may suitablybe substantially convex, preferably in the shape of a shallow dish. Itmay be made of a resilient material in order to help maintain reducedpressure above the hydrogel after suction has been applied. The backingsheet may be provided with a layer of a medically acceptablepressure-sensitive adhesive on at least the periphery thereof forattachment of the backing sheet to the skin around the wound to betreated, but this is not essential since the present invention does notseek to maintain a reduced pressure at the wound surface.

The screen provides support for the hydrogel and a reservoir for storageof wound fluid so that it is not necessary continuously to drain fluidfrom the dressing. Accordingly, the screen is suitably formed from aresilient or rigid, porous water-absorbent material.

Suitable screen materials comprise or consist essentially of opencellular foams formed of a hydrophilic polymeric material, such aspolyurethane or polyester. Such foams are advantageous, since they areresilient and therefore they are compressed by the application ofvacuum, but do not collapse to the extent that porosity and liquidpermeability are lost. Typically the thickness of the screen material isfrom about 1 mm to about 30 mm, for example from about 5 mm to about 15mm. Other materials that may be used for the screen include freeze-driedsponges of biopolymers, and nonwoven textile materials. The screen maybe made up of a plurality of layers of the same or different materials.

The term “hydrogel” refers to medically acceptable aqueous gel of one ormore macromolecular substances that form a gel with water underphysiological conditions of temperature and pH. Such hydrogelspreferably have the ability to swell and absorb fluid while maintaininga strong integral structure. Preferably, the hydrogel is substantiallyinsoluble in water under physiological conditions, whereby the hydrogelis not washed away by the wound fluid.

Exemplary insoluble hydrogels include certain cross-linked polyacrylategels, polyacrylamide gels, polyurethane gels, cross-linked biopolymergels, carboxymethyl cellulose gels, hydroxyethyl cellulose gels, hydroxypropyl methyl cellulose, and gels formed from vinyl alcohols, vinylesters, vinyl ethers and mixtures thereof. Cross-linked biopolymer gelsinclude calcium alginate gels, cross-linked hyaluronate gels, pectingels, galactomannan gels, chitosan gels, cross-linked gelatin, andmixtures thereof. Other suitable gels include gels formed from andcarboxy vinyl monomers, meth (acrylic) acrylamide, N-vinylpyrrolidone,acylamidopropane sulphonic acid, PLURONIC (Registered Trade Mark) (blockpolyethylene glycol, block polypropylene glycol) polystyrene-, maleicacid, NN-dimethylacrylamide, diacetone acrylamide, acryloyl morpholine,and mixtures thereof. In certain embodiments, the hydrogel comprises ahydrogel material of the kind described in WO-A-0007638.

The hydrogel may be resorbable, that is to say it may be fully brokendown and reabsorbed in vivo in the mammalian body.

The hydrogels may be cross-linked, and the cross-linking may be eithercovalent or ionic. The hydrogel material may comprise from about 5% toabout 99% by weight, for example from about 10% to about 90% by weightof water based on the weight of the hydrogel. The hydrogel material mayfurther comprise from 5 to 50% by weight on a dry weight basis of one ormore humectants such as glycerol.

The thickness of the hydrogel layer is suitably from about 0.5 mm toabout 20 mm, for example from about 1 mm to about 5 mm. As alreadynoted, the hydrogel layer is continuous and bonded to the backing sheetaround its periphery so as to for an airtight barrier between the woundand the suction aperture in the backing sheet. The hydrogel layer istherefore continuous, not permeable or semipermeable to air, and has noopen area. The hydrogel layer permits the passage of liquids but notgas. The application of suction to the suction aperture thereforeresults in a pressure differential across the hydrogel layer that drivesliquid migration through the hydrogel layer into the screen.

The area of the hydrogel layer may vary according to the type and sizeof wound, but is typically from about 1 cm² to about 500 cm², forexample from about 4 cm² to about 100 cm².

The hydrogel layer may be separate from the screen layer. In theseembodiments the hydrogel layer may be supported by the screen layer orby a separate, rigid or semi-rigid apertured support layer so that thehydrogel layer does not collapse under suction. In certain embodiments,a part or all of the hydrogel layer partially penetrates the screenlayer. That is to say, the wound facing side of the screen layer issuitably impregnated with the hydrogel material. This improves fluidtransfer between the hydrogel layer and the screen layer, and providesmechanical reinforcement and support to the hydrogel layer. Typically,from about 10% to about 90%, for example from about 20% to about 75%,suitably about 50% of the thickness of the screen layer may beimpregnated with the hydrogel. These embodiment may be made, forexample, by first impregnating the surface of the screen layer with aliquid formulation of a hydrogel prepolymer, followed by cross-linkingthe prepolymer in situ to form the hydrogel. For example, the hydrogelprepolymer could be a sodium alginate solution that is cross-linked insitu by treatment with a calcium salt. Alternatively, the hydrogelprepolymer could be a fluid mixture of a diisocyanate cappedpolyurethane prepolymer and a suitable cross-linking agent such as aprimary amine that reacts in situ to form a polyurethane hydrogel. Inyet other embodiments, the hydrogel prepolymer could be a solution ofacrylamide monomers as described in WO-A-2004052415 that is polymerizedin situ by irradiation with UV light. In yet other embodiments, thehydrogel prepolymer could be a gelatin solution that is cross-linked insitu by treatment with a cross-linking agent such as glutaraldehyde.These techniques are also useful for forming the airtight seal betweenthe hydrogel layer and the periphery of the backing sheet.

Any of the component materials making up the screen and/or the hydrogellayer may comprise an antimicrobial material. Suitable antimicrobialagents may be selected from the group consisting of antiseptics andantibiotics and mixtures thereof. Suitable antibiotics peptideantimicrobials (e.g. defensins, Magainin, synthetic derivatives of them)tetracycline, penicillins, terramycins, erythromycin, bacitracin,neomycin, polymycin B, mupirocin, clindamycin and mixtures thereof.Suitable antiseptics include silver sulfadiazine, chlorhexidine,povidone iodine, triclosan, other silver salts and colloidal silver,sucralfate, quaternary ammonium salts and mixtures thereof. Theantimicrobial agents are suitably present in the screen or the hydrogelin an amount of from about 0.001% to about 2% by weight based on theweight of the screen or the hydrogel, respectively.

Alternatively or additionally, the hydrogel may comprise one or moretherapeutic agents that promote wound healing. Exemplary therapeuticagents include an antioxidant or free radical scavenger such as VitaminC (ascorbic acid), retinoids such as Vitamin A, Vitamin E, ORC (whichhas been shown to have antioxidant properties), hydroquinones,benzimidazoles, antioxidant-grafted polysaccharides such as thosedescribed in U.S. Pat. No. 5,612,321, aniline or acridine dyes, ormixtures or combinations thereof. The antioxidant or free radicalscavengers are suitably present in the hydrogel in an amount of fromabout 0.001% to about 2% by weight based on the weight of the screen orthe hydrogel, respectively.

Alternatively or additionally, the hydrogel layer may contain one ormore growth factors selected from the group consisting of plateletderived growth factor (PDGF), fibroblast growth factor (FGF),transforming growth factor beta (TGF-P), epidermal growth factor (EGF),vascular endothelial growth factor (VEGF) and insulin-like growth factor(IGF), and mixtures thereof. The growth factors are suitably present inthe hydrogel in an amount of from about 0.0001% to about 0.1% by weightbased on the weight of the screen or the hydrogel, respectively.

The wound facing surface of the dressing is suitably protected by aremovable protective cover sheet. The cover sheet is typically formedfrom flexible thermoplastic material. Suitable materials include but arenot limited to polyesters and polyolefins. Suitably, the hydrogel-facingsurface of the cover sheet is a release surface. That is to say, asurface that is only weakly adherent to the hydrogel and any adhesive onthe hydrogel or the backing sheet to assist peeling of the dressing fromthe cover sheet. For example, the cover sheet may be formed from anon-adherent plastic such as a fluoropolymer, or it may be provided witha release coating such as a silicone or fluoropolymer release coating.

Suitably, the wound dressing according to the invention is sterile andpackaged in a microorganism-impermeable container.

The systems according to the present invention further comprise a sourceof suction in fluid communication with the aperture for applying asub-atmospheric pressure inside the dressing in the space between thehydrogel layer and the backing sheet.

The source of suction may be an electric or manually operated air pump.It may be located remote from the dressing, or it may be attached to orform part of the dressing as in the case of micro pumps such as thosedescribed in WO-A-2007030601.

For more heavily exuding wounds it may be desirable to drain wound fluidfrom the dressing at least intermittently. In these embodiments thesystem according to the present invention suitably further comprises awound fluid collection vessel intermediate (and in fluid communicationwith) the backing sheet aperture and the suction source for collectingwound fluid drained from the screen. The wound fluid collection vesselmay itself form part of the suction source, for example it may be avacuum bottle. The wound fluid collection vessel may contain anantimicrobial agent.

In the methods according to the invention, the dressings according tothe invention are applied to a wound, for example an exuding wound, withthe hydrogel layer covering the wound. The dressings may be secured tothe intact skin around the wound by a peripheral adhesive-coated marginof the backing sheet, in conventional fashion. Alternatively oradditionally the dressings may be secured in place by a suitablesecondary dressing. A suction device, such as a vacuum pump or a squeezebulb is used to withdraw air from the space between the backing sheetand the hydrogel layer, through the aperture in the backing sheet. Thereduced pressure in this space may result in some compression of thescreen in the said space, but the screen is sufficiently resilient tomaintain its porosity and absorbency under this compression. Suitably,the screen is sufficiently resilient so that elastic recovery of thescreen continues to maintain a negative pressure in the space for sometime after the suction means has been withdrawn and the aperture closed.Thus, it may not be necessary to apply continuous suction to thedressing. The pressure differential across the hydrogel layer promotesmigration of wound fluid through the hydrogel layer and into theabsorbent screen layer. This liquid may then be drained from the screenlayer and into a suitable collection vessel by continued application ofsuction.

The term “vacuum” or “suction” here and elsewhere in the presentspecification refers to any pressure below ambient atmospheric pressure.Suitably, the step of applying a vacuum includes lowering the pressurein the space between the backing sheet and the hydrogel layer to anabsolute value of from about 0.1 bar to about 0.95 bar, suitably fromabout 0.5 bar to about 0.9 bar and typically to an absolute value offrom about 0.75 bar to about 0.85 bar. The vacuum may be static ordynamic. The vacuum may be applied continuously or intermittently.

It will be appreciated that any feature or embodiment of the presentinvention that is described in relation to any one aspect of theinvention is equally applicable to any other embodiment of the inventionmutatis mutandis.

Specific embodiments of the present invention will now be discussedfurther, by way of example, with reference to the accompany drawings, inwhich:

FIG. 1 shows a perspective view of a wound dressing system according tothe present invention; and

FIG. 2 shows a partial longitudinal cross section through the wounddressing of the system of FIG. 1.

Referring to FIG. 1, the wound dressing system according to theinvention comprises a wound dressing 1 according to the invention, avacuum bottle 2 for providing suction, and a tube 3 connecting thevacuum bottle to the dressing. The dressing 1 comprises a flexiblebacking sheet 4 formed of substantially impermeable, thermoformedthermoplastic substantially in the shape of a tray having dimensionsabout 10 cm×10 cm×1 cm. An aperture 5 in the backing sheet 4, isconnected to the tube 3. In alternative embodiments, the backing sheetmay be provided with a flange around its periphery, and a layer ofmedically acceptable pressure-sensitive adhesive on the flange forattachment of the backing sheet to the skin around the wound beingtreated. The wound dressing 1 further comprises a screen 6, which inthis particular embodiment is a layer of open-cellular hydrophilicpolyurethane foam approximately 1 cm thick substantially filling thebacking sheet. A suction manifold 8 in the form of a branched tube isembedded in the polyurethane foam and connected to the aperture 5 forefficient drainage of fluid from the foam layer. The wound facingsurface of the foam layer 6 is impregnated with a continuous medicallyacceptable hydrogel layer 7 that extends across and bonds with theperiphery of the backing sheet 4 to form an airtight seal. The hydrogellayer may, for example, be a polyurethane hydrogel or a polyacrylamidehydrogel.

The wound facing hydrogel layer in this and the other embodiments may beprotected by a releasable protective sheet (not shown) prior to use. Thedressing is usually packaged in a microorganism impermeable container,and is sterilised e.g. by gamma irradiation.

In use, the dressing is applied to a wound with the hydrogel layerfacing and covering the wound. The vacuum bottle 2 is partiallyevacuated with a pump (not shown) to achieve a desired, subatmosphericpressure above the hydrogel layer. Wound fluid is drawn through thehydrogel layer and through tube 3 into bottle 2, which thereby servesalso as a wound fluid collection vessel.

The above embodiment has been described for purpose of illustration.Many other embodiments falling within the scope of the accompanyingclaims will be apparent to the skilled reader.

The invention claimed is:
 1. A system for the treatment of a tissue sitewith negative pressure, the system comprising: a source of suction; anda wound dressing in fluid communication with the source of suction andcomprising: a backing sheet having an aperture for communication withthe source of suction; a screen layer; and a hydrogel layer covering awound facing side of the screen layer and bonded around a periphery tothe backing sheet to form an air-impermeable hydrogel layer, wherein thehydrogel layer is resorbable.
 2. The system of claim 1, wherein thescreen layer is rigid or semi-rigid to support the air-impermeablehydrogel layer.
 3. The system of claim 1, wherein the screen layer is aporous foam.
 4. The system of claim 1, wherein the screen layer is anopen-cell foam.
 5. The system of claim 1, further comprising a suctionmanifold connected to the source of suction and located inside thescreen layer.
 6. The system of claim 1, wherein the hydrogel layercomprises a biopolymer.
 7. The system of claim 1, wherein the hydrogellayer comprises oxidized reduced cellulose (ORC).
 8. The system of claim1, wherein the hydrogel layer comprises silver.
 9. A method of treatinga wound, comprising: applying a dressing comprising a backing sheet, ascreen layer and a hydrogel layer, the hydrogel layer covering a woundfacing side of the screen layer and bonded around a periphery to thebacking sheet to form an air-impermeable hydrogel layer, wherein thehydrogel layer is resorbable; and applying reduced pressure to anon-wound-facing surface of the hydrogel layer.
 10. The method of claim9, wherein the screen layer is rigid or semi-rigid to support theair-impermeable hydrogel layer.
 11. The method of claim 9, wherein thescreen layer is a porous foam.
 12. The method of claim 9, wherein thescreen layer is an open-cell foam.
 13. The method of claim 9, furthercomprising a suction manifold connected to the source of suction andlocated inside the screen layer.
 14. The method of claim 9, wherein thehydrogel layer comprises a biopolymer.
 15. The method of claim 9,wherein the hydrogel layer comprises oxidized reduced cellulose (ORC).16. The method of claim 9, wherein the hydrogel layer comprises silver.